lamost sky survey --site limitations and survey planningnaw/cambridge-beijing/... · 2008. 12....

Dec 12 2008 C. Liu@KIAA-Cambridge Joint Workshop on Near-Field Cosmology and Galactic Archeology

LAMOST Sky SurveyLAMOST Sky Survey ----Site limitations and survey planningSite limitations and survey planning

Chao Liu, Licai DengNational Astronomical Observatories, CAS

Heidi NewbergRensselaer Polytechnic Institute


Overview

• Site limitations that strictly constrain the survey and subsequently scientific goals of LAMOST– weather and sky brightness

• Proposal of Milky Way stellar survey with LAMOST (LEGUE)– c.f. a previous talk of Heidi Newberg


LAMOST optics


LAMOST Facts

• Aperture: ~4m• Type: Schmidt, Alt-Az, Meridian• Focal length: 20m• Field of view: 5 degree diameter• Size of focus plane: 1.75m• Sky coverage: Dec>-10degrees, 1.5hours

around meridian• Wavelength: 370~900nm, R=1000/2000• Number of fibers: 4000, 16 spectrographs with

250 fibers each


Location

• Location: – E117◦34’ N40◦23’

• Elevation: – ~900m


Weather

• Climate: Continental Monsoon– Summer: rainy, cloudy, humid– Winter: cold, dry, clear– Spring: windy

BATC weather logs from 2004.1 to 2007.9The redder dots representsthe better weather nights


Footprint of the planningLAMOST Survey of Milky Way stars

24000 sqdegs in total (blue area are double counted)


Extinction Coefficient

Y. Liu et al. 2003

k=0.3

BATC historical data from 1995-2001


Seeing

Y. Liu et al. 2003

BATC is a Schmidt telescope with CCDimage resolution of1.7arcsec/pixel

A small telescope isbeing used to monitorthe seeing for LAMOST


Sky Brightness

• Moon• Light pollution from nearby cities

– turning worse due to economy development• Climate factors• Air pollutions• Scattering

– surrounding ground of MA


Moon contribution in sky brightness

Y. Liu et al. 2003

BATC monitor data pointing to the North pole from 1995-2001

In order to estimate the sky brightness for any position of the sky at any time, we needa model to describe the sky brightness as a function ofmoon phase and angular separation between moon and sky position etc.


Modeling the sky brightness

• Krisciunas & Schaefer 1991– based on observation at Mauna Kea

where αis the phase angle of the moon, ρ is the angular separation betweenthe observation sky position and the moon, f(ρ) is the atmospheric scattering function, which is the sum of Rayleigh and Mie scattering, Z is the zenith distance of the sky position. k is the extinction coeffecient, and Bmoon is the model surface brightness contributed of the moon


Testing the model with sky brightness data at the north pole in Xinglong observed by BATC from 1995-2004. The data includes bad weather which is not suitable for observation as well.

The model sky brightness is ~0.3mag deeperthan the data.

Difficulties for the model applying on Xinglong:

-different atmospheric scattering -extinction coefficient -weather conditions


-BATC sky brightness data is turning worse in the future years.

-Fiber mag of sky brightness at 20.5mag would be 17.6~18.4mag

-This will constrain the scientific goal This will constrain the scientific goal of LAMOSTof LAMOST

The dark night sky brightness

ObjType skybr=20.5 skybr=19.5 skybr=19

S/N=3 S/N=10 S/N=3 S/N=10 S/N=3 S/N=10

Pointed src 21.8 20.5 21.3 20 21.1 19.8Elliptical 21.5 20.2 21 19.7 20.8 19.5Spiral 21.7 20.4 21.2 19.9 20.9 19.6

Limited mag with 90min exposure at 550nm (Xue Y. 2008)


Sky CoverageDEC=-10~70degs (vignetting is significant when DEC>70deg )902 plates, each of which is ~20sqdegs: 605 in higher Galactic latitude (|b|>20) and 297 in lower Galactic latitude (|b|<20)

The strategy of the survey is to assign the dark and grey nights to fainter objects at high latitude and assign the bright nights to brighter objects at low latitude


Observe high latitude at grey/dark nights

580 of 605 plates will be observed from 2010-2014;totally 2609 observed platesAverage repeat times:~4.5

Sky brightness

Repeat times


Observe low latitude at bright nights

204 of 297 plates will be observed from 2010-2014;totally 609 observed platesAverag repeat times~3

Sky brightness

Repeat times


LAMOST is a meridian telescope and cannot point to any position, thus the sky footprint of one-year survey is discontinuous. The following plots are survey footprints in 2010


Maximum number of spectra (5-year plan)• Total observed plates: ~3218

– High latitude(|b|>20, g<20): ~2609– Low latitude(|b|<20, B<16):~609

• Total number of spectra/year– High latitude: 2609/5*3500~1.8 x 106

– Low Latitude: 609/5*3500*2.5~1.1 x 106

– Totally ~1.9 x 106

• A 5-year plan: ~1.45 x 107 spectra– 9 x 106 in high latitude (stars, QSOs and Galaxies)– 5.5 x 106 in low latitude (stars, HI, HII, CO, ...)


The LAMOST Survey of Milky Way Stars(LEGUE)• Basics

– 2.5 million stellar spectra in 5 years• 17<g<20 at high latitude• B<16 at low latitude

– use dark/grey nights at high latitude with a quarter of the fibers pointing to stellar objects

– bright nights at low latitude with most fibers pointing to stellar objects

– R=5000 grating used at low latitude


The LAMOST Survey of Milky Way Stars(LEGUE)• Scientific goals

– Substructure, formation and evolution of the Galactic stellar halo

– kinematics, formation history, chemical and dynamical evolution of the disk population


Substructure of the Milky Way

Dec 12 2008 C. Liu@KIAA-Cambridge Joint Workshop on Near-Field Cosmology and Galactic ArcheologyIvezic et al. 2008


Outstanding problems• The observation data is more complicated than models, how do we

compare them?• What the dark matter potential of the Milky Way looks like?• How many stellar components are there in the Milky Way, and how

to describe them?• What is the detail structure of the Milky Way’s disks? How is it

related to Monoceros/Canis Major?• How many dwarf galaxies are merged to create the Milky Way, and

when?• Study the frequency of metallicity as a function of position in the

Milky Way, favoring detection of rare, low metallicity stars.• Disentangle the components of the Galactic disks and possible tidal

debris therein• Describe the chemical evolution of the Galactic disks.• How do we utilize all of the partial chemical, kinematical, and spatial

information at the same time?


Suggestions for target selection

• Understand the limitation of the telescope: instrument, site and time allocation

• Identify the grand challenge science goals that can be accomplished, and design a grand survey that can be analyzed statistically.

• How to consistently select target objects from both SDSS footprint and non-SDSS


Thanks!

lamost sky survey --site limitations and survey planningnaw/cambridge-beijing/... · 2008. 12....

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